This invention relates to a method for preparing a luminescent screen, as for a cathode-ray tube, which screen exhibits markedly improved adherence to its supporting surface during the processing thereof.
In preparing a luminescent screen by the slurry-direct photographic process, as described, for example, in U.S. Pat. No. 3,406,068 to H.B. Law, a glass surface, such as the inner surface of the glass faceplate for a color television picture tube, is coated with an aqueous slurry comprising a photosensitizable binder (photobinder), a photosensitizer therefor, and particles of phosphor material. The phosphor-photobinder coating is dried and then exposed to a light pattern, as by exposure through an apartment mask, to produce regions of greater and regions of lesser solubility in the coating. The exposed coating is developed by removing the more-soluble regions of the coating, as by spraying and/or flushing the coating with water or aqueous solution under pressure. The adherence of the retained less-soluble regions of the coating to the glass surface is important and is particularly critical during the development step. The loss of even a small part of the less-soluble regions, which should constitute portions of the screen, requires the screen to be scrapped.
It is known that the adherence of the coating to a clean glass surface can be improved by applying to the glass surface a very thin precoating of a water-soluble polymeric material prior to applying the coating. See, for example, Canadian Pat. No. 602,838 to W. W. Slobbe and U.S. Pat. No. 3,481,733 to L. W. Evans. In a typical process, the surface of a glass panel is washed with an aqueous ammonium bifluoride composition, rinsed with deionized water, then rinsed with a dilute solution of PVA (polyvinyl alcohol) of about 0.2 to 0.5 weight percent concentration and then dried. It is theorized that a very thin, perhaps monomolecular, precoating of PVA remains on the glass surface, which precoating improves the adherence of a subsequently-applied phosphor-photobinder coating. It is not apparent why such a thin precoating improves the adherence of the subsequently-applied coating. However, it has been observed that aging the PVA precoating in air at room temperature, or acidifying the PVA precoating with a mineral acid, further improves the adherence of the subsequently-applied phosphor coating. When heavier screen weights in the range of 4 to 6 milligrams phosphor per square centimeter (mg/cm.sup.2) are desired, improved adherence can be achieved by employing a heavy precoating of light-exposed dichromate-sensitized PVA or other photosensitive organic colloid with or without phosphor particles present. Another method, disclosed in U.S. Pat. No. 3,966,474 to S. A. Harper, employs a thick adherent precoating consisting essentially of water-insoluble, organic, polymeric particles, preferably deposited from an aqueous emulsion thereof.
It is the practice, when the phosphor-photobinder coating is being dried on automatic or semiautomatic machines, to heat the coating and the glass support to about 50.degree. to 55.degree. C. When the dry coating is being exposed to the light pattern, the temperature of the glass support has dropped somewhat to about 40.degree. to 45.degree. C. This practice will be referred to herein as the "hot-application" process. It is often desirable to maintain lower temperatures in the glass support while the coating is being dried and is being exposed in order to reduce energy consumption and/or to improve the placement of the retained, less-soluble regions of the coating. A "cool-applicatio" process, in which the coating is dried at temperatures of about 30.degree. to 38.degree. C. and is exposed at temperatures of about 25.degree. to 33.degree. C., has been found to be practical. However, prior precoatings have not been as effective in improving the adherence of the coating in the cool-application process as they are in the hot-application process.